1. Field of the Invention
The present invention relates to an MPEG audio/video decoder. More specifically, the present invention relates to an apparatus and operating method for synchronizing the presentation of audio and video frames.
2. Description of the Related Art
Standardization of recording media, devices and various aspects of data handling, such as audio recording, storage and playback, is highly desirable for continued growth of this technology and its applications. One compression standard which has attained wide spread use for compressing and decompressing video information is the moving pictures expert group (MPEG) standard for audio and video encoding and decoding. The MPEG standard is defined in International Standard ISO/IEC 11172-1, "Information Technology--Coding of moving pictures and associated audio for digital storage media at up to about 1.5 Mbit/s", Parts 1, 2 and 3, First edition 1993-08-01 (hereinafter referred to as MPEG International Standard).
An MPEG decoding system receives compressed data and reconstructs the compressed data into data frames for presentation at appropriate presentation times. The decoding system models the delay between audio and video performance devices as a zero delay even though actual delays are imposed, variable between different devices, and may be extended by post-processing or output operations. Audio and video presentations are typically synchronized by displaying a video frame instantaneously at a scheduled presentation time. A first audio sample is presented instantaneously at the scheduled presentation time and subsequent audio samples are presented in sequence at an audio sampling rate.
The MPEG International Standard addresses the problem of combining one or more data streams from video and audio sources with timing information to form a single information stream. The presentation of audio frames and the presentation of video frames are related by system timing signals. The system timing signals produce synchrony between the audio frames and corresponding video frames so long as the audio and video frames are controlled by the same timer. However, in practice the audio presentation and the video presentation in a personal computer environment are typically controlled by two different timers so that a mismatch in frequency between the two timers is likely. After some duration of time has elapsed the frequency mismatch between the timers inevitably results in a loss of synchrony between the audio and video presentations.
What is needed is a technique for enforcing audio/video synchrony.